Positive electrode active material for sodium secondary battery, positive electrode for sodium secondary battery, and sodium secondary battery

Abstract

To provide a positive electrode active material for a sodium secondary battery, which can realize a sodium secondary battery having a large charge / discharge capacity and an energy density equal to or greater than that of a lithium secondary battery.SOLUTION: A positive electrode active material for a sodium secondary battery, includes a composite metal oxide represented by the formula of NaxMn1-y-zM1yM2zO2 (1), and has a high initial discharge capacity compared to an initial charge capacity. In the formula (1), M1 represents Ni or Fe, M2 represents Co, Ti, Mg, Cu or Al, x is 0.95 or less, y is 0.1 to 0.5, and z is 0 to 0.1. A positive electrode active material for a sodium secondary battery includes a composite metal oxide represented by the formula (1), and has a higher initial discharge capacity than the initial charge capacity.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a sodium secondary battery, and more particularly to a positive electrode active material for a sodium secondary battery containing a transition metal oxide.

Background Art

[0002] Sodium secondary batteries can be composed of materials that are abundant in supply and inexpensive, and it is expected that by putting them into practical use, a large amount of large-scale power sources can be supplied. On the other hand, sodium secondary batteries have insufficient practicality due to their low energy density. One of the reasons for the low energy density of sodium secondary batteries is that the capacity of the positive electrode material of sodium secondary batteries is insufficient.

[0003] Patent Document 1 describes a sodium secondary battery positive electrode material composed of a sodium-containing transition metal oxide represented by the composition formula Na x (Mn y Co 1-y-z Ni z )O 2 and having a composition ratio of 0.5 < x ≦ 1.0, 0.60 ≦ y ≦ 0.80, 0.05 ≦ z ≦ 0.20, 0.05 ≦ 1 - y - z ≦ 0.25. This sodium secondary battery positive electrode material has a specific composition as described above, and further provides a sodium secondary battery having a higher charge-discharge capacity than conventional sodium secondary batteries by controlling the firing temperature of the raw material mixture and the atmosphere during firing.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The present invention aims to provide a positive electrode active material for a sodium secondary battery that can realize a sodium secondary battery having a large charge / discharge capacity and an energy density equal to or higher than that of a lithium secondary battery. Conventional positive electrode active materials for sodium secondary batteries still have insufficient charge / discharge capacity to achieve an energy density equal to or higher than that of a lithium secondary battery. [Means for solving the problem]

[0006] The present invention provides the following aspects. [Aspect 1] Formula Na x Mn 1-y-z M1 y M2 z O 2 (1) [In the formula, M1 represents Ni or Fe, M2 represents Co, Ti, Mg, Cu or Al, x represents 0.95 or less, y represents 0.1 to 0.5, and z represents 0 to 0.1.] The composite metal oxide is represented by A positive electrode active material for sodium secondary batteries, which has a higher initial discharge capacity compared to the initial charge capacity.

[0007] [Aspect 2] The positive electrode active material for a sodium secondary battery according to Aspect 1, wherein x is 0.2 to 0.8, preferably 0.3 to 0.7, and more preferably 0.4 to 0.6.

[0008] [Aspect 3] The positive electrode active material for a sodium secondary battery according to Aspect 1 or 2, wherein y is 0.15 to 0.3, preferably 0.15 to 0.25.

[0009] [Aspect 4] The positive electrode active material for a sodium secondary battery according to any one of Aspects 1 to 3, wherein z is 0.01 to 0.08, preferably 0.03 to 0.07.

[0010] [Aspect 5] The positive electrode active material for a sodium secondary battery according to any one of Aspects 1 to 4, wherein an initial discharge capacity is at least 150% higher, preferably 200 to 400%, and more preferably 250 to 350% higher than an initial charge capacity.

[0011] [Aspect 6] The positive electrode active material for a sodium secondary battery according to any one of Aspects 1 to 5, which has a higher charge / discharge capacity from the second time onwards than the initial charge capacity.

[0012] [Aspect 7] The positive electrode active material for a sodium secondary battery according to any one of Aspects 1 to 6, having an initial discharge capacity of more than 210 mAh / g, preferably 220 to 280 mAh / g.

[0013] [Embodiment 8] The positive electrode active material for a sodium secondary battery according to any one of Embodiments 1 to 7, comprising a mixture obtained by firing a precipitate from an aqueous solution of a metal to be oxidized and a sodium compound.

[0014] [Aspect 9] After discharging the sodium secondary battery, the formula Na x Mn 1-y-z M1 y M2 z O 2 (3) [In the formula, x represents a value greater than 1. M1, M2, y, and z are as defined above.] 9. The positive electrode active material for a sodium secondary battery according to any one of Aspects 1 to 8, comprising a composite metal oxide represented by the following formula:

[0015] [Aspect 10] The positive electrode active material for a sodium secondary battery of Aspect 9, wherein x is greater than 1 and is equal to or less than 2.

[0016] [Aspect 11] A positive electrode for a sodium secondary battery comprising the positive electrode active material for a sodium secondary battery of any one of Aspects 1 to 10.

[0017] [Aspect 12] The positive electrode for a sodium secondary battery according to Aspect 11, which has been subjected to a charging treatment at a voltage exceeding 4.2 V, for example, 4.4 to 4.5 V, based on the sodium electrode potential.

[0018] [Aspect 13] The positive electrode for a sodium secondary battery according to Aspect 11 or 12, comprising a sodium compound.

[0019] [Aspect 14] The positive electrode for a sodium secondary battery of Aspect 13, wherein the sodium compound is contained in the positive electrode in an amount of 1 to 50 mol %, preferably 3 to 40 mol %, and more preferably 5 to 30 mol %, based on the number of moles of the oxidizable metal portion in the positive electrode active material.

[0020] [Aspect 15] The positive electrode for a sodium secondary battery of Aspect 13 or 14, wherein the sodium compound includes sodium hydroxide or sodium carbonate.

[0021] [Aspect 16] Formula Mn 1-y-z M1 y M2 z (2) [In the formula, M1 represents Ni or Fe, M2 represents Co, Ti, Mg, Cu or Al, x represents 0.95 or less, y represents 0.1 to 0.5, and z represents 0 to 0.1.] contacting an aqueous solution containing the metals of the composition represented by the formula (I) with a precipitating agent in the presence of oxygen to obtain a precipitate; and mixing the precipitate with a sodium compound and calcining the mixture; The present invention relates to a method for producing a positive electrode active material for a sodium secondary battery.

[0022] [Aspect 17] A method for producing a positive electrode for a sodium secondary battery, comprising forming an electrode mixture containing the positive electrode active material for a sodium secondary battery obtained by the production method of Aspect 16 into a shape of a positive electrode.

[0023] [Aspect 18] A sodium secondary battery having the positive electrode for sodium secondary batteries according to any one of Aspects 11 to 15.

[0024] [Aspect 19] A method for producing the sodium secondary battery of Aspect 18, comprising charging at a voltage exceeding 4.2 V, for example, 4.4 to 4.5 V, based on the sodium electrode potential.

[0025] [Aspect 20] The positive electrode active material after discharge is represented by the formula Na x Mn 1-y-z M1 y M2 z O 2 (3) [In the formula, x represents a value greater than 1. M1, M2, y, and z are as defined above.] 20. The sodium secondary battery of embodiment 18 or 19, comprising a composite metal oxide represented by the formula:

[0026] [Aspect 21] The sodium secondary battery of Aspect 20, wherein the x is greater than 1 and not greater than 2, preferably 1.05 to 1.7, and more preferably 1.1 to 1.5.

[0027] [Aspect 22] The positive electrode for a sodium secondary battery of Aspect 13 or 14, wherein the sodium compound includes pentasodium ferrate.

[0028] [Aspect 23] The positive electrode for a sodium secondary battery of Aspect 13 or 14, wherein the sodium compound is supported on the surface of the positive electrode active material.

[0029] [Aspect 24] The positive electrode for a sodium secondary battery of Aspect 23, wherein the sodium compound includes sodium hydroxide, sodium carbonate, or pentasodium ferrate.

[0030] [Aspect 25] A sodium secondary battery having the positive electrode for sodium secondary batteries according to any one of Aspects 22 to 24. Effect of the Invention

[0031] According to the present invention, it is possible to provide a positive electrode active material for a sodium secondary battery, which can realize a sodium secondary battery having a large charge / discharge capacity and an energy density equal to or greater than that of a lithium secondary battery. [Brief description of the drawings]

[0032] [Figure 1] 1 is an exploded view showing a schematic configuration of a sodium secondary battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] The scope of the present invention is not limited to the embodiments described herein, and various modifications can be made without departing from the spirit of the present invention. In addition, when multiple upper and lower limit values ​​are described for a specific parameter, any upper and lower limit values ​​can be combined to form a suitable numerical range.

[0034] <Cathode active material> The positive electrode active material for a sodium secondary battery of the present invention has the formula Na x Mn 1-y-z M1 y M2 z O 2 (1) The positive electrode active material comprises a composite metal oxide represented by the formula (1). Formula (1) represents the chemical structure of the positive electrode active material before the first charge of the sodium secondary battery. In a preferred embodiment, the positive electrode active material comprises a composite metal oxide represented by the formula (1). In the formula (1), Na represents sodium. Mn represents manganese. M1 includes a metal other than an alkali metal element. In one embodiment, M1 includes at least one selected from the group consisting of nickel (Ni) and iron (Fe). M2 includes at least one selected from the group consisting of cobalt (Co), titanium (Ti), magnesium (Mg), copper (Cu), and aluminum (Al). Also, O represents oxygen.

[0035] In a preferred embodiment of formula (1), M1 is made of Ni. In another embodiment, M1 is made of Fe. Preferred examples of combinations of M1 and M2 include Ni and Co, Ni and Ti, Ni and Mg, Ni and Cu, and Ni and Al. Other examples of combinations of M1 and M2 include Fe and Co, Fe and Ti, Fe and Mg, Fe and Cu, and Fe and Al.

[0036] The Na atomic ratio x in the above formula is 0.95 or less, preferably 0.2 to 0.8, more preferably 0.3 to 0.7, and further preferably 0.4 to 0.6. If x exceeds 0.95, sodium tends to remain in the positive electrode active material, which may result in insufficient charge / discharge capacity.

[0037] The atomic ratio y of the metal M1 in the above formula is 0.1 to 0.5, preferably 0.15 to 0.3, and more preferably 0.15 to 0.25. The Mn atomic ratio 1-yz in the above formula is 0.5 to 0.95, preferably 0.6 to 0.9, and more preferably 0.7 to 0.85. When y and 1-yz are within the above ranges, the discharge capacity of the positive electrode active material is improved.

[0038] The atomic ratio z of M2 in the above formula is 0 to 0.1, preferably 0.01 to 0.08, and more preferably 0.03 to 0.07. M2 is not an essential component, but when it is contained in the above amount, the stability of the crystal structure of the composite metal oxide is improved.

[0039] The positive electrode active material of the present invention has a characteristic that the initial discharge capacity is higher than the initial charge capacity, and the initial discharge capacity is 150% or more, preferably 200 to 400%, and more preferably 250 to 350% higher than the initial charge capacity.

[0040] In one embodiment, the positive electrode active material of the present invention has an initial discharge capacity of 150 to 400 mAh / g, preferably 200 to 300 mAh / g, more preferably 220 to 280 mAh / g, for example, a value higher than 210 mAh / g.

[0041] In another embodiment, the positive electrode active material of the present invention may have an initial discharge capacity of 170 to 270 mAh / g, preferably 180 to 260 mAh / g, and more preferably 190 to 250 mAh / g.

[0042] Furthermore, the positive electrode active material of the present invention can exhibit a charge / discharge capacity that is higher in the second and subsequent charge / discharge cycles than the initial charge capacity.

[0043] The method for producing a positive electrode active material of the present invention includes contacting an aqueous solution containing a metal to be oxidized (i.e., a metal element selected from the group consisting of metal elements excluding alkali metal elements) with a precipitant to obtain a precipitate, mixing the precipitate with a sodium compound, and calcining the mixture. In this specification, the precipitate is sometimes referred to as a "precipitate from the aqueous solution of the metal to be oxidized."

[0044] The metal to be oxidized includes manganese and metal M1. M1 includes metals other than alkali metal elements. In one embodiment, M1 includes at least one selected from the group consisting of Ni and Fe. The metal to be oxidized may include metal M2. M2 includes at least one selected from the group consisting of Co, Ti, Mg, Cu, and Al. Specific examples of the use of M1 alone and the use of M1 and M2 in combination are the same as those described above.

[0045] The composition ratio of the metal to be oxidized contained in the aqueous solution is preferably expressed by the formula Mn 1-y-z M1 y M2 z (2) In the formula (2), the values ​​of y, z and 1-yz are the same as defined above.

[0046] An aqueous solution containing the metal to be oxidized can be obtained by using compounds such as chlorides, nitrates, acetates, formates, oxalates, etc. as raw materials and dissolving them in water. In addition, when a raw material that is difficult to dissolve in water is used, that is, when an oxide, hydroxide, or metal material is used as the raw material, the raw material can be dissolved in an acid such as hydrochloric acid, sulfuric acid, or nitric acid, or an aqueous solution of these acids, to obtain an aqueous solution containing the metal to be oxidized.

[0047] Precipitants include LiOH (lithium hydroxide), NaOH (sodium hydroxide), KOH (potassium hydroxide), Li 2 CO 3 (Lithium carbonate), Na 2 CO 3 (Sodium carbonate), K 2 CO 3(Potassium carbonate), (NH 4 ) 2 CO 3 (Ammonium carbonate) and (NH 2 ) 2 One or more compounds selected from the group consisting of CO (urea) may be used, one or more hydrates of the compounds may be used, or the compounds and hydrates may be used in combination. It is also preferable to dissolve these precipitants in water and use them in the form of an aqueous solution. The concentration of the compound in the aqueous solution of the precipitant is about 0.5 to 10 mol / L, preferably about 1 to 8 mol / L. It is also preferable to use NaOH as the precipitant, and more preferably, it is an aqueous NaOH solution obtained by dissolving NaOH in water. An example of an aqueous solution of the precipitant is ammonia water, which may be used in combination with the aqueous solution of the compound.

[0048] Examples of methods for contacting an aqueous solution containing a metal to be oxidized with a precipitant include a method of adding a precipitant (including an aqueous solution of a precipitant) to an aqueous solution containing a metal to be oxidized, a method of adding an aqueous solution containing a metal to be oxidized to an aqueous solution of a precipitant, and a method of adding an aqueous solution containing a metal to be oxidized and a precipitant (including an aqueous solution of a precipitant) to water. When contacting an aqueous solution containing a metal to be oxidized with a precipitant, it is preferable to contact the metal to be oxidized with oxygen. Among the above contact methods, a method of adding an aqueous solution containing a metal to be oxidized to an aqueous solution of a precipitant under an air atmosphere is preferable.

[0049] In the present invention, a slurry containing a precipitate can be obtained by the above contact. The precipitate contains the metal to be oxidized. The precipitate from the aqueous solution of the metal to be oxidized preferably includes the precipitate that has been contacted with oxygen in the aqueous solution of the metal to be oxidized.

[0050] Next, the slurry is subjected to solid-liquid separation to recover the precipitate. Any method may be used for the solid-liquid separation, but from the viewpoint of operability, a method of solid-liquid separation such as filtration is preferably used, and a method of volatilizing the liquid by heating such as spray drying may also be used. In addition, the recovered precipitate may be washed, dried, etc. The precipitate obtained after solid-liquid separation may have excess precipitant components attached thereto, and the components can be reduced by washing. As a washing liquid used in washing, water is preferably used, and a water-soluble organic solvent such as ethanol or acetone may also be used. In addition, drying may be performed by heating and drying, and may be performed by blowing and vacuum drying, etc. In the case of performing heating and drying, it is usually performed at 50 to 300°C, preferably about 100 to 200°C. In addition, washing and drying may be performed two or more times.

[0051] In the present invention, the precipitate obtained as described above is mixed with a sodium compound and fired to obtain a positive electrode active material. The mixing ratio of the sodium compound and the precipitate is such that the relationship between sodium and the oxidized metal satisfies the relationship of formula (1). Since sodium is easily volatilized when heated, sodium may be mixed in an amount that is in excess of the relationship of formula (1) relative to the oxidized metal.

[0052] The sodium compound may be one or more compounds selected from the group consisting of sodium hydroxide, sodium chloride, sodium nitrate, sodium peroxide, sodium sulfate, sodium bicarbonate, sodium oxalate, sodium carbonate, sodium phosphate, and pentasodium ferrate, and may also be hydrates thereof. The mixing method may be either dry mixing or wet mixing, but dry mixing is preferred from the viewpoint of simplicity. The mixing device may be a stirrer mixer, a V-type mixer, a W-type mixer, a ribbon mixer, a drum mixer, a ball mill, etc.

[0053] The calcination is carried out at a temperature of usually 400 to 1200°C, preferably 600 to 1000°C, more preferably 700 to 900°C, depending on the type of sodium compound used. If the calcination temperature is too low, sodium tends to remain in the positive electrode active material, resulting in insufficient charge / discharge capacity, whereas if the temperature is too high, the crystal structure of the positive electrode active material may be destroyed. As a result of the calcination, a composite metal oxide of formula (1) is produced.

[0054] The time for maintaining the firing temperature is usually 0.1 to 20 hours, and preferably 0.5 to 10 hours. The firing atmosphere may be air, oxygen, nitrogen, argon, or a mixture of these gases, but air is preferred from the viewpoint of simplicity.

[0055] The mixture of the precipitate and the sodium compound may be calcined under a vacuum state in which oxygen is blocked. The vacuum state here generally refers to a vacuum degree of about 10^5 Pa to 10^-5 Pa, preferably about 10^2 Pa to 10^-1 Pa, and more preferably about 10^-1 Pa to 10^-5 Pa.

[0056] The positive electrode active material obtained by firing may or may not be washed with a solvent such as distilled water. If the firing is performed in a vacuum state where oxygen is blocked and the positive electrode active material is not washed, the positive electrode active material may contain more sodium ions than the sodium ions released during the first charge.

[0057] The positive electrode active material obtained as described above may be pulverized using a ball mill or a jet mill, or the pulverization and firing may be repeated two or more times. The positive electrode active material may be washed or classified as necessary.

[0058] The composition ratio of the obtained positive electrode active material can be identified, for example, by ICP (inductively coupled plasma atomic absorption spectrometry).

[0059] <Positive electrode> The positive electrode of the present invention contains the positive electrode active material. The positive electrode may contain a sodium compound in addition to the positive electrode active material. This allows a larger amount of sodium ions to be supplied to the electrolyte than the sodium ions released by the positive electrode active material during initial charging, and the energy density of the sodium secondary battery is further improved. The sodium compound used is one that releases sodium ions. Specific examples of the sodium compound are as described above, and preferred sodium compounds include sodium hydroxide, sodium carbonate, and pentasodium ferrate. The sodium compound is contained in the positive electrode in an amount of 1 to 50 mol%, preferably 3 to 40 mol%, and more preferably 5 to 30 mol%, based on the number of moles of the oxidized metal portion in the positive electrode active material. The "oxidized metal portion in the positive electrode active material" refers to the oxidized metal portion represented by formula (2).

[0060] The positive electrode is useful as a positive electrode in a sodium secondary battery and can be used as the positive electrode of the battery. From the viewpoint of obtaining a sodium secondary battery that provides a larger potential difference, i.e., a sodium secondary battery with a higher energy density, it is preferable to use the positive electrode as the positive electrode in the sodium secondary battery.

[0061] The positive electrode of the present invention can be produced by forming an electrode mixture containing the positive electrode active material and, if necessary, a sodium compound into the shape of a positive electrode. The electrode mixture generally contains a binder and, if necessary, a conductive agent. The electrode mixture may be supported on an electrode current collector and formed.

[0062] The method for producing the positive electrode includes, for example, mixing the positive electrode active material, a binder, a sodium compound if necessary, and a conductive material if necessary to obtain an electrode mixture, and forming the obtained electrode mixture into a shape of a positive electrode. The electrode mixture may be obtained by spraying a sodium compound solution onto the positive electrode active material, drying the solvent if necessary, and mixing the binder and the conductive material if necessary. The electrode mixture may also be obtained by preparing a liquid in which the positive electrode active material, a sodium compound, and a solvent are mixed, drying the solvent if necessary, and mixing the binder and the conductive material if necessary. These methods for producing a positive electrode are safe and do not require the handling of metallic sodium as a raw material, and can be performed using simple steps and equipment.

[0063] Examples of the conductive agent include carbon materials such as natural graphite, artificial graphite, cokes, and carbon black. Examples of the binder include thermoplastic resins, specifically fluororesins such as polyvinylidene fluoride (hereinafter sometimes referred to as "PVDF"), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, and tetrafluoroethylene-perfluorovinyl ether copolymers; and polyolefin resins such as polyethylene and polypropylene.

[0064] When the positive electrode is used as the positive electrode of a secondary battery, Al, Ni, stainless steel, etc. can be used as the electrode current collector.

[0065] Examples of methods for forming the electrode mixture include a method of pressurizing the electrode mixture itself, or a method of forming the electrode mixture into a paste using an organic solvent, applying the paste onto an electrode current collector, and then pressing the electrode current collector after drying to fix the electrode current collector. When forming the electrode mixture into a paste, a slurry consisting of an electrode active material, a conductive agent, a binder, and an organic solvent is prepared. Examples of organic solvents include amine-based solvents such as N,N-dimethylaminopropylamine and diethyltriamine; ether-based solvents such as ethylene oxide and tetrahydrofuran; ketone-based solvents such as methyl ethyl ketone; ester-based solvents such as methyl acetate; and aprotic polar solvents such as dimethylacetamide and N-methyl-2-pyrrolidone. Examples of methods for applying the electrode mixture to an electrode current collector include slit die coating, screen coating, curtain coating, knife coating, gravure coating, and electrostatic spraying.

[0066] <Negative electrode> The negative electrode used in the present invention may be an electrode used in a sodium ion battery, and may be an electrode containing a negative electrode active material capable of absorbing and releasing sodium, such as a hard carbon, a soft carbon, a natural graphite, an artificial graphite, a coke, a carbon black, a pyrolytic carbon, a carbon fiber, a fired organic polymer compound, or other carbon materials, a layered phosphorus compound, a composite oxide of phosphorus and titanium, such as Maxene, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Mo, Pd, Ag, Cd, In, Sn, Sb, W, Pb, and Bi, or an alloy, oxide, halide, or chalcogenide using these elements.

[0067] Carbon materials can be obtained, for example, by carbonizing phenolic resins (novolac-type phenolic resins, resol-type phenolic resins, etc.), epoxy resins (bisphenol-type epoxy resins, novolac-type epoxy resins, etc.), aniline resins, bismaleimide resins, benzoxazine resins, etc. Among them, phenolic resins are characterized by a structure with developed three-dimensional cross-linking, and it is presumed that the carbon material obtained by carbonizing such resins is also a carbon material having a unique structure with developed three-dimensional cross-linking derived from this characteristic, and is considered to be preferable from the viewpoint of increasing the charge / discharge capacity.

[0068] A sodium secondary battery has the property of releasing sodium ions from the positive electrode active material during charging and incorporating sodium ions into the positive electrode active material during discharging. It is recognized that the discharge capacity of the positive electrode active material of the present invention can be higher than the theoretical capacity corresponding to the case where x in formula (1) is 1. In such a case, the positive electrode active material after discharging the sodium secondary battery has a capacity of 100% by weight, which is expressed by the formula Na x Mn 1-y-z M1 y M2 z O 2 (3) [In the formula, x represents a value greater than 1. M1, M2, y, and z are as defined above.] The above x is preferably more than 1 and not more than 2, more preferably 1.05 to 1.7, and even more preferably 1.1 to 1.5. Formula (3) represents the chemical structure of the positive electrode active material after discharging the sodium secondary battery.

[0069] Since the capacity of the positive electrode active material of the present invention increases during the first discharge compared to the first charge, it is necessary to absorb a larger amount of sodium ions during the first discharge than the sodium ions released during the first charge. Therefore, the high charge / discharge capacity inherent to the positive electrode active material can be exhibited by means of, for example, pre-containing a sodium compound in the positive electrode, increasing the electrolyte concentration of the electrolyte solution to a higher level than in the past, or pre-doping the negative electrode with sodium ions.

[0070] When the electrolyte concentration of the electrolyte solution is made higher than that of the conventional one or when the negative electrode is doped with sodium ions in advance, the electrolyte concentration of the electrolyte solution or the amount of sodium doped into the negative electrode is preferably set so that the equivalent ratio of sodium to the metal to be oxidized is 1:1, or so that sodium is in excess of the equivalent ratio of 1:1 relative to the metal to be oxidized, based on the entire sodium secondary battery.

[0071] The method for doping the negative electrode with sodium is not particularly limited, and examples thereof include (i) a method in which metallic sodium is attached to a portion of the negative electrode current collector where there is no negative electrode material, and a local cell is formed by injecting the metallic sodium, and sodium is doped into the negative electrode active material; (ii) a method in which metallic sodium is formed on the negative electrode material by vapor deposition or sputtering, and sodium is doped into the negative electrode material by a solid-phase reaction; (iii) a method in which sodium is electrochemically doped into the negative electrode before the battery is constructed in an electrolyte; and (iv) a method in which metallic sodium is added and mixed during the preparation of the composite powder used in the present invention, thereby doping sodium into the negative electrode material.

[0072] From the viewpoint of improving the energy density of a sodium secondary battery, the amount of sodium doped in the negative electrode is 60% by weight or less, preferably 1 to 40% by weight, and more preferably 5 to 30% by weight, based on the negative electrode active material. If the amount of sodium doped exceeds 60% by weight based on the negative electrode active material, the energy density of the entire cell may decrease, which is not preferable.

[0073] The negative electrode mixture contains the negative electrode active material, and, if necessary, the binder, the conductive agent, etc. Examples of the negative electrode current collector include Al, Cu, Ni, stainless steel, etc., and Al or Cu is preferable because it is difficult to form an alloy with sodium and is easy to process into a thin film. The method of supporting the negative electrode current collector is the same as that of the positive electrode described above, and includes a method of pressurizing and molding, a method of applying a paste using a solvent or the like to the negative electrode current collector, drying, and then pressing to fix it.

[0074] <Sodium secondary battery> The sodium secondary battery of the present invention has the positive electrode and the negative electrode. The sodium secondary battery of the present invention can be manufactured by obtaining an electrode group by stacking or winding a positive electrode, a separator, and a negative electrode in this order, storing the electrode group in a container such as a battery can, and impregnating the electrode group with an electrolytic solution made of an organic solvent containing an electrolyte. The positive electrode or the negative electrode may be an electrode current collector carrying an electrode mixture.

[0075] Examples of the shape of the electrode group include a shape in which the cross section of the electrode group cut in a direction perpendicular to the winding axis is a circle, an ellipse, a rectangle, a rectangle with rounded corners, and the like.

[0076] The shape of the battery may be, for example, a paper type, a coin type, a cylindrical type, a square type, or the like.

[0077] Fig. 1 is an exploded view showing a schematic configuration of a sodium secondary battery according to one embodiment of the present invention. The sodium secondary battery shown in Fig. 1 is a coin-type sodium secondary battery, and is made up of an electrode plate assembly using the positive electrode 4, an electrolyte (not shown), and a battery case that houses these. The electrode plate assembly is made up of a sheet-shaped positive electrode 4, a sheet-shaped negative electrode 6, and a sheet-shaped separator 5 that provides insulation between the positive electrode 4 and the negative electrode 6.

[0078] These are stacked together with a leaf spring 2, a spacer 3, a gasket 7, and the like, which are commonly used in the field of sodium secondary batteries, and are housed in battery cases 1, 8.

[0079] As the separator that can be used in the sodium secondary battery, for example, a material having a form of a porous film, a nonwoven fabric, a woven fabric, etc., made of a material such as a polyolefin resin, such as polyethylene or polypropylene, a fluororesin, or a nitrogen-containing aromatic polymer, can be used. In addition, a single layer or a laminate separator using two or more of these materials may be used. As the separator, for example, the separator described in JP-A-2000-30686 and JP-A-10-324758 can be mentioned. The thickness of the separator is preferably as thin as possible while maintaining mechanical strength, in that the volumetric energy density of the battery increases and the internal resistance decreases. The thickness of the separator is generally preferably about 5 to 200 μm, more preferably about 5 to 40 μm. From the viewpoint of ion permeability, the separator preferably has an air permeability of 50 to 300 sec / 100 cc, more preferably 50 to 200 sec / 100 cc, in terms of the air permeability according to the Gurley method. The porosity of the separator is usually 30 to 80% by volume, and preferably 40 to 70% by volume. The separator may be a laminate of separators having different porosities.

[0080] In the electrolyte that can be used in the sodium secondary battery, examples of the organic solvent include propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, 1,2-di(methoxycarbonyloxy)ethane and other carbonates; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3 Ethers such as 3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, and 2-methyltetrahydrofuran; esters such as methyl formate, methyl acetate, and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, and 1,3-propane sultone; or organic solvents as mentioned above further containing fluorine substituents can be used. Usually, two or more of these are mixed and used as the organic solvent.

[0081] The electrolyte of the electrolytic solution is not particularly limited, but a salt generally used in sodium secondary batteries can be used. For example, NaPF 6 , NaBF 4 , NaClO 4 , NaTiF 4 , NaVF 5 , NaAsF, NaSbF 6 , NaCF 3 SO 3 , Na(C 2 F 5 SO 2 ) 2 N, NaB(C 2 O 4 ) 2 , NaB 10 Cl 10 , NaB 12 Cl 12 , NaCF 3 COO, Na 2 S2 O 4 , NaNO 3 , Na 2 SO 4 , NaPF 3 (C 2 F 5 ) 3 , NaB(C 6 F 5 ) 4 , and Na(CF 3 SO 2 ) 3 It is possible to use salts such as C. It is to be noted that the above salts may be used alone or in combination of two or more.

[0082] Of these, sodium hexafluorophosphate (NaPF 6 ), sodium perchlorate (NaClO 4 ), sodium tetrafluoroborate (NaBF 4 ) is preferably used, and NaPF 6 Preferably, NaPF 6 By using this as a salt, the effect of improving the discharge capacity and cycle life of the positive electrode and improving the cycle life of the negative electrode is enhanced.

[0083] From the viewpoint of improving the energy density of a sodium secondary battery, the electrolyte concentration of the electrolytic solution is 1.0 mol / kg or more, preferably 1.3 to 1.5 mol / kg, and more preferably 1.2 to 1.3 mol / kg.

[0084] The sodium secondary battery is operated at a voltage of 4.2V or less based on the sodium electrode potential. If it is operated at a voltage higher than this, the cycle durability may decrease due to inconveniences such as exceeding the oxidation resistance pressure of the electrolyte solvent. On the other hand, it has been revealed that the energy density of the sodium secondary battery can be further increased by exposing the positive electrode of the present invention to a high voltage. In other words, from the viewpoint of increasing the capacity, it is preferable that the positive electrode active material or positive electrode of the present invention is temporarily exposed to a high voltage of 4.2V or more.

[0085] Specifically, when charging the sodium secondary battery, positive electrode active material or positive electrode of the present invention, it is preferable to apply a voltage of 4.2 V or more, for example, 4.4 to 4.5 V based on the sodium electrode potential at least once. This can further increase the energy density of the sodium secondary battery and the charge / discharge capacity of the positive electrode active material or positive electrode. The increase rate of the charge / discharge capacity (mAh / g) of the positive electrode active material or positive electrode is 5 to 25%, preferably 10 to 15%. After the positive electrode active material has been subjected to a high voltage, the sodium secondary battery can be operated at a rated voltage of, for example, 4.2 V or less, thereby preventing deterioration of cycle characteristics due to high voltage while maintaining improved performance. EXAMPLES

[0086] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

[0087] <Examples 1 to 17> (1) Synthesis of complex metal oxides Manganese chloride, cobalt chloride, nickel chloride, iron chloride, titanium trichloride, magnesium chloride, and copper chloride were dissolved in pure water in a prescribed ratio to prepare a metal solution. Sodium hydroxide solution was prepared by dissolving 10 g of sodium hydroxide in 50 ml of pure water. The metal solution was dropped into the sodium hydroxide solution in an air atmosphere to form a precipitate, which was then contacted with oxygen. The precipitate was filtered from the slurry and washed with pure water.

[0088] The precipitate and sodium hydroxide were weighed out in amounts such that the metal and sodium contained in the precipitate were in an equivalent ratio of 1:1, and pure water was added and mixed uniformly. The obtained slurry was dried at 120°C. The obtained powder was pre-fired in air at 600°C for 5 hours and crushed in a mortar. The obtained powder was pre-fired at 750°C for 5 hours, crushed in a mortar, washed with water, and dried. The obtained powder was main fired in air at 900°C, crushed in a mortar, washed with water, and dried. In this way, a composite metal oxide was synthesized. The compounding ratio (molar ratio) of the metals is shown in Table 1.

[0089] The composition ratios of the composite metal oxides synthesized in Examples 13 to 15 and 17 were measured by ICP (inductively coupled plasma atomic absorption spectrometry). 0.45 Mn 0.8 Ni 0.15 Co 0.05 O 2 (Example 13) Na 0.55 Mn 0.8 Ni 0.15 Ti 0.05 O 2 (Example 14) Na 0.60 Mn 0.8 Ni 0.15 Mg 0.05 O 2 (Example 15) and Na 0.40 Mn 0.8 Ni 0.15 Cu 0.05 O 2 (Example 17).

[0090] (2) Preparation of positive and negative electrodes A composite metal oxide as a positive electrode active material or a phenol resin carbide as a negative electrode active material, acetylene black (manufactured by Denki Kagaku Co., Ltd.) as a conductive material, and polyfluorotetraethylene (PTFE, "F104" (trade name) manufactured by Daikin Industries, Ltd.) as a binder were weighed out so as to have a composition of electrode active material:conductive material:binder = 8:1:1 (weight ratio), and were placed in a bottle and shaken to mix. The mixture was crushed in a mortar while applying shear force to form a sheet with a thickness of about 100 μm. The obtained sheet was punched out to a diameter of 13 mm to obtain a positive electrode or a negative electrode.

[0091] (4) Preparation of Na-doped anode A metallic sodium foil having a thickness of 20 μm and a diameter of 5 mm was pressed onto the dried negative electrode disk in a glove box.

[0092] (5) Fabrication of sodium secondary battery The obtained positive and negative electrodes, electrolyte, separator, gasket, case, and other commonly used parts were prepared to assemble a sodium secondary battery cell. The ratio of the weight of the positive and negative electrodes was adjusted to 1:0.8. An exploded perspective view of the cell is shown in Figure 1.

[0093] (Electrolyte A) NaPF was dissolved in a 1:1 mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at a specified concentration. 6 An electrolyte solution containing:

[0094] (Electrolyte B) NaClO was dissolved in a 1:1 mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at a given concentration. 4 An electrolyte solution containing:

[0095] Next, a charge / discharge test was performed on the cell under the following conditions using a charge / discharge tester to determine its characteristics. The measurement items and results are shown in Tables 1 and 2.

[0096] Current: 0.lmAh Mode: CCCVCC(0.05C cutoff)CC Temperature: 35℃ Positive voltage range: 1.25~4.2V

[0097] <Example 18> Using a charge / discharge test device, the sodium secondary battery of Example 1 was subjected to one cycle of charge / discharge under the following conditions.

[0098] Current: 0.lmAh Mode: CCCVCC(0.05C cutoff)CC Temperature: 35℃ Positive voltage range: 1.25~4.4V

[0099] Thereafter, the sodium secondary batteries that had been subjected to the high voltage were subjected to the determination of their battery characteristics in the same manner as in Example 1. The measurement items and the measurement results are shown in Tables 1 and 2.

[0100] <Example 19> A composite metal oxide was synthesized and a sodium secondary battery was fabricated in the same manner as in Example 1, except that sodium carbonate was used instead of sodium hydroxide, and the characteristics were tested. The results are shown in Tables 1 and 2.

[0101] <Comparative Example 1> A sodium secondary battery was fabricated in the same manner as in Example 1, except that a composite metal oxide of Mn:Ni:Fe=1:1:1 (manufactured by Beijing Dangsheng Co., Ltd.) was used as the positive electrode active material, and the characteristics were tested. The results are shown in Tables 1 and 2.

[0102] <Comparative Example 2> Sodium carbonate (Na 2 CO 3 ) powder and manganese(II,III) oxide (Mn 3 O 4 ) and nickel manganese cobalt hydroxide (NiMnCo)OH 2 were weighed out so that the molar ratios of Na:(Mn+Co+Ni) were 1:1, and the molar ratios of Mn:Ni:Co were 8:1.5:0.5. These were mixed in a mortar, then filled into an alumina crucible, and fired in an electric furnace at 900°C in air for 10 hours. The mixture was then naturally cooled in the electric furnace to obtain a composite metal oxide.

[0103] A sodium secondary battery was produced in the same manner as in Example 1, except that the obtained composite metal oxide was used as the positive electrode active material, and the characteristics were tested. The results are shown in Tables 1 and 2.

[0104] <Control Example 1> The positive electrode active material was a composite metal oxide (manufactured by Beijing Dangsheng Co., Ltd.) with a ratio of Mn:Ni:Co=1:1:1, the negative electrode was graphite (manufactured by BTR Co., Ltd.), and the electrolyte was LiPF 6 A lithium secondary battery was fabricated and its characteristics were tested in the same manner as in Example 1, except that a 1 mol / L solution of (EC:DEC=1:1) was used. The results are shown in Tables 1 and 2.

[0105] (Electrolyte C) LiPF at a given concentration was dissolved in a 1:1 mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). 6 An electrolyte solution containing:

[0106] <Control Example 2> As the positive electrode active material, LFP (lithium iron phosphate (LiFePO 4 ), Gelon), graphite (BTR), and LiPF 6 A lithium secondary battery was fabricated and its characteristics were tested in the same manner as in Example 1, except that a 1 mol / L solution of (EC:DEC=1:1) was used. The results are shown in Tables 1 and 2.

[0107] <Control Example 3> A lithium secondary battery was fabricated in the same manner as in Comparative Example 2, except that silicon (Si)-doped graphite (manufactured by BTR) was used as the negative electrode, and the characteristics were tested. The results are shown in Tables 1 and 2.

[0108] [Table 1] *1: Phenol resin carbide *2: Sodium-doped phenolic resin carbide *3: Anode made of metallic sodium with a thickness of 250 μm and a diameter of 14 mm

[0109] [Table 2]

[0110] From the measurement results shown in Table 2, it can be seen that a sodium secondary battery using the positive electrode active material of the present invention has an energy density equal to or higher than that of a lithium secondary battery using a ternary material or an LFP lithium secondary battery.

[0111] <Examples 20 to 22> Preparation of sodium carbonate-containing positive electrode The metal portion to be oxidized in the composite metal oxide (positive electrode active material) produced in Example 1 was mixed with sodium carbonate at a predetermined compounding ratio. The obtained mixture, acetylene black (manufactured by Denki Kagaku Co., Ltd.) as a conductive material, and polyfluorotetraethylene (PTFE, "F104" (trade name) manufactured by Daikin Industries, Ltd.) as a binder were weighed out so as to have a composition of mixture:conductive material:binder=8:1:1 (weight ratio), and were placed in a bottle and shaken to mix. The mixture was crushed in a mortar while applying a shearing force to form a sheet having a thickness of about 100 μm. The obtained sheet was punched out to a diameter of 13 mm to obtain a positive electrode.

[0112] A sodium secondary battery was fabricated in the same manner as in Example 1 except that the obtained positive electrode was used, and the characteristics were tested. The results are shown in Table 3.

[0113] <Example 23> Sodium carbonate and the composite metal oxide (positive electrode active material) produced in Example 1 were dispersed in ethanol. This dispersion was dried by spray drying to obtain particles carrying sodium carbonate on the surface of the active material. At that time, the amount of sodium carbonate mixed was adjusted so that the compounding ratio of the oxidized metal portion in the positive electrode active material to the sodium carbonate was a predetermined compounding ratio. A positive electrode was obtained in the same manner as in Example 20, except that the obtained mixture was used.

[0114] A sodium secondary battery was fabricated in the same manner as in Example 1 except that the obtained positive electrode was used, and the characteristics were tested. The results are shown in Table 3.

[0115] [Table 3] *1: Phenol resin carbide *2: Sodium carbonate was dispersed in ethanol together with the active material of Example 1, and the resulting dispersion was spray-dried to be supported on the surface of the active material.

[0116] <Example 24> A sodium secondary battery was prepared in the same manner as in Example 11, and charged and discharged three times under the same conditions. At the third charge and discharge, the charge generation capacity and the discharge generation capacity were measured. After that, the sodium secondary battery in the discharged state was disassembled and the positive electrode was taken out. It was washed with ethanol, and the positive electrode mixture was dissolved with nitric acid. The residue was filtered from the dissolved material to prepare a sample solution, and its composition was analyzed using ICP. The results are shown in Table 4.

[0117] <Example 25> A sodium secondary battery was prepared in the same manner as in Example 17, and charged and discharged three times under the same conditions. At the third charge and discharge, the charge generation capacity and the discharge generation capacity were measured. After that, the sodium secondary battery in the discharged state was disassembled and the positive electrode was taken out. It was washed with ethanol, and the positive electrode mixture was dissolved with nitric acid. The residue was filtered from the dissolved material to prepare a sample solution, and its composition was analyzed using ICP. The results are shown in Table 4.

[0118] <Control Example 4> A sodium secondary battery was produced in the same manner as in Example 23, except that a composite metal oxide of Mn:Ni:Fe=1:1:1 (manufactured by Beijing Tosho Co., Ltd.) was used as the positive electrode active material, and the charge / discharge generation capacity was measured. After that, the positive electrode active material was taken out from the sodium secondary battery in a discharged state, and its composition was analyzed using ICP. The results are shown in Table 4.

[0119] [Table 4]

[0120] From the experimental results shown in Table 4, it can be understood that the discharge capacity of a sodium secondary battery using the positive electrode active material of the present invention can be increased beyond the theoretical capacity, and that the positive electrode active material after discharge has the chemical structure shown in formula (3).

[0121] <Example 26> A composite metal oxide was synthesized in the same manner as in Example 1, except that manganese chloride and iron chloride were used in amounts such that the molar ratio of Mn to Fe was 8:2.

[0122] The composite metal oxide (positive electrode active material) and Na 5 FeO 4 The mixture was dispersed in ethanol in an amount such that the molar ratio was 1:0.1. The dispersion was dried by spray drying to form Na on the surface of the active material. 5 FeO 4 A positive electrode was obtained in the same manner as in Example 20, except that the obtained mixture was used.

[0123] A sodium secondary battery was fabricated in the same manner as in Example 1 except that the obtained positive electrode was used, and the characteristics were tested. The results are shown in Table 5.

[0124] <Example 27> Manganese chloride and iron chloride were dissolved in pure water in amounts such that the molar ratio of Mn to Fe was 8:2 to prepare a metal solution. Sodium hydroxide solution was prepared by dissolving 10 g of sodium hydroxide in 50 ml of pure water. The metal solution was added dropwise to the sodium hydroxide solution in an air atmosphere to form a precipitate, which was then contacted with oxygen. The precipitate was filtered from the slurry and washed with pure water.

[0125] The precipitate and sodium hydroxide were weighed in an amount such that the metal and sodium contained in the precipitate were in an equivalent ratio of 1:1, and pure water was added and mixed uniformly. The obtained slurry was dried at 120°C. The obtained powder was pre-fired at 600°C for 5 hours in a vacuum and crushed in a mortar. The obtained powder was pre-fired at 750°C for 5 hours and crushed in a mortar. The obtained powder was main-fired at 900°C in a vacuum, crushed in a mortar, and dried. In this way, a composite metal oxide was synthesized. The compounding ratio (molar ratio) of the metals is shown in Table 1. A composite metal oxide was synthesized in the same manner as in Example 1, except that the above-mentioned 100% 1,2-dichloro- ...

[0126] A positive electrode was obtained in the same manner as in Example 20, except that the produced composite metal oxide (positive electrode active material) was used.

[0127] A sodium secondary battery was fabricated in the same manner as in Example 1 except that the obtained positive electrode was used, and the characteristics were tested. The results are shown in Table 5.

[0128] [Table 5]

[0129] *1: Phenol resin carbide ※2:Na 5 FeO 4 was dispersed in ethanol together with the positive electrode active material, and the dispersion was spray-dried to be supported on the surface of the positive electrode active material. [Explanation of symbols]

[0130] 1, 8 cases 2. Leaf spring 3 Spacer 4 Positive electrode 5. Separator 6 negative electrode 7 Gasket

Claims

1. formula <h2 style=";text-align:left;direction:ltr">Na<h2 style=";text-align:left;direction:ltr"> ｘ <h2 style=";text-align:left;direction:ltr"> 4Mn<h2 style=";text-align:left;direction:ltr"> １－ｙ-z <h2 style=";text-align:left;direction:ltr"> 11<h2 style=";text-align:left;direction:ltr"> ｙ <h2 style=";text-align:left;direction:ltr"> 12<h2 style=";text-align:left;direction:ltr"> ｚ <h2 style=";text-align:left;direction:ltr"> O<h2 style=";text-align:left;direction:ltr"> ２ <h2 style=";text-align:left;direction:ltr"> (1) [In the formula, M1 represents Ni or Fe, M2 represents Co, Ti, Mg, Cu or Al, x represents 0.95 or less, y represents 0.1 to 0.5, and z represents 0 to 0.1.] The composite metal oxide is represented by A positive electrode active material for sodium secondary batteries, which has a higher initial discharge capacity compared to the initial charge capacity.

2. The positive electrode active material for a sodium secondary battery according to claim 1, wherein x is 0.2 to 0.

8.

3. The positive electrode active material for a sodium secondary battery according to claim 2, wherein y is 0.15 to 0.

3.

4. The positive electrode active material for a sodium secondary battery according to claim 3, wherein z is 0.01 to 0.

08.

5. The positive electrode active material for a sodium secondary battery according to any one of claims 1 to 4, wherein an initial discharge capacity is 150% or more higher than an initial charge capacity.

6. The positive electrode active material for a sodium secondary battery according to any one of claims 1 to 4, wherein the second and subsequent charge / discharge capacities are higher than the initial charge capacity.

7. The positive electrode active material for a sodium secondary battery according to any one of claims 1 to 4, wherein an initial discharge capacity is higher than 210 mAh / g.

8. The positive electrode active material for a sodium secondary battery according to any one of claims 1 to 4, comprising a mixture of a precipitate from an aqueous solution of a metal to be oxidized and a sodium compound, and the mixture is fired.

9. After discharging the sodium secondary battery, the formula is <h2 style=";text-align:left;direction:ltr">Na<h2 style=";text-align:left;direction:ltr"> ｘ <h2 style=";text-align:left;direction:ltr"> 4Mn<h2 style=";text-align:left;direction:ltr"> １－ｙ-z <h2 style=";text-align:left;direction:ltr"> 11<h2 style=";text-align:left;direction:ltr"> ｙ <h2 style=";text-align:left;direction:ltr"> 12<h2 style=";text-align:left;direction:ltr"> ｚ <h2 style=";text-align:left;direction:ltr"> O<h2 style=";text-align:left;direction:ltr"> ２ <h2 style=";text-align:left;direction:ltr"> (3) [In the formula, x represents a value greater than 1. M1, M2, y, and z are as defined above.] The positive electrode active material for a sodium secondary battery according to any one of claims 1 to 4, comprising a composite metal oxide represented by the formula:

10. The positive electrode active material for a sodium secondary battery according to claim 9 , wherein the x is greater than 1 and is equal to or less than 2.

11. A positive electrode for a sodium secondary battery comprising the positive electrode active material for a sodium secondary battery according to any one of claims 1 to 4.

12. The positive electrode for a sodium secondary battery according to claim 11, which has been subjected to a charging treatment at a voltage exceeding 4.2 V based on a sodium electrode potential.

13. The positive electrode for a sodium secondary battery according to claim 11 , comprising a sodium compound.

14. The positive electrode for a sodium secondary battery according to claim 13, wherein the sodium compound is contained in the positive electrode in an amount of 1 to 50 mol % based on the number of moles of the oxidizable metal portion in the positive electrode active material.

15. The positive electrode for a sodium secondary battery according to claim 13 , wherein the sodium compound includes sodium hydroxide or sodium carbonate.

16. formula <h2 style=";text-align:left;direction:ltr">4Mn<h2 style=";text-align:left;direction:ltr"> １－ｙ－ｚ <h2 style=";text-align:left;direction:ltr"> 11<h2 style=";text-align:left;direction:ltr"> ｙ <h2 style=";text-align:left;direction:ltr"> 12<h2 style=";text-align:left;direction:ltr"> ｚ <h2 style=";text-align:left;direction:ltr"> (2) [In the formula, M1 represents Ni or Fe, M2 represents Co, Ti, Mg, Cu or Al, x represents 0.95 or less, y represents 0.1 to 0.5, and z represents 0 to 0.1.] contacting an aqueous solution containing the metals of the composition represented by the formula (I) with a precipitating agent in the presence of oxygen to obtain a precipitate; and mixing the precipitate with a sodium compound and calcining the mixture; The present invention relates to a method for producing a positive electrode active material for a sodium secondary battery.

17. A method for producing a positive electrode for a sodium secondary battery, comprising forming an electrode mixture containing the positive electrode active material for a sodium secondary battery obtained by the method of claim 16 into a shape of a positive electrode.

18. A sodium secondary battery comprising the positive electrode for sodium secondary batteries according to claim 11.

19. The method for producing a sodium secondary battery according to claim 18, comprising charging at a voltage exceeding 4.2 V based on a sodium electrode potential.

20. The positive electrode active material after discharge has the formula <h2 style=";text-align:left;direction:ltr">Na<h2 style=";text-align:left;direction:ltr"> ｘ <h2 style=";text-align:left;direction:ltr"> 4Mn<h2 style=";text-align:left;direction:ltr"> １－ｙ-z <h2 style=";text-align:left;direction:ltr"> 11<h2 style=";text-align:left;direction:ltr"> ｙ <h2 style=";text-align:left;direction:ltr"> 12<h2 style=";text-align:left;direction:ltr"> ｚ <h2 style=";text-align:left;direction:ltr"> O<h2 style=";text-align:left;direction:ltr"> ２ <h2 style=";text-align:left;direction:ltr"> (3) [In the formula, x represents a value greater than 1. M1, M2, y, and z are as defined above.] The sodium secondary battery according to claim 18, comprising a composite metal oxide represented by the formula:

21. The sodium secondary battery according to claim 20 , wherein x is greater than 1 and is equal to or less than 2.

22. The positive electrode for a sodium secondary battery according to claim 13 or 14, wherein the sodium compound includes pentasodium ferrate.

23. 15. The positive electrode for a sodium secondary battery according to claim 13 or 14, wherein the sodium compound is supported on a surface of the positive electrode active material.

24. The positive electrode for a sodium secondary battery according to claim 23, wherein the sodium compound comprises sodium hydroxide, sodium carbonate, or pentasodium ferrate.

25. A sodium secondary battery comprising the positive electrode for a sodium secondary battery according to any one of claims 22 to 24.

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